Plant vs Animal Fats

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Just like with animals, plants store their fats in cells (for plants, in a plastid called elaioplast). Unlike with animals, plant cells have cell walls, and these cell walls are not digestible by humans because we lack the enzymes to do so. The cell walls are made out of cellulose, hemicelluloses and pectin. These are all “fiber” compounds (ie indigestible).
these cell walls ... can reduce the contact of amylolytic enzymes with starch granules and lower-energy availability for nonruminant animals by acting either as a physical barrier or by increasing the viscosity of the digesta. – “Endosperm Cell-Wall Composition, Thickness, and Integrity” J.L. Black, in Encyclopedia of Grain Science, 2004

As with cell membranes, plant cell walls also have incorporated proteins. It is expectable that because these proteins are digestible by humans, we may digest small parts in the plant cell walls leading to gaps that might compromise the structure and make the contents within the disrupted cell wall semi-available. However, in comparing an animal fat cell to a plant fat cell, the fat content in the animal cell is readily available once the digestible cell membrane is ruptured by enzymes and bile, whereas, with plant cells, if they reach the stomach intact, there is no guarantee any of their content will be digested since their is no guarantee the cell wall will be sufficiently disrupted.

There are some caveats to this:

  1. Bacteria in the large intestine may break down the plant cells and turn the contents into a useable fuel like butyrate, but this depends upon a person’s personal microbiota.
  2. When a seed is germinating (aka sprouting), it will contain enzymes to break down the endosperm (area of fuel carrying plant cells) which can make seeds more digestible
    • “Enzymes Released from the Aleurone during Modification” D.J. Mares, … G.B. Fincher, in Encyclopedia of Grain Science, 2004
      • there is little or no glucanase activity in ungerminated barley grain
      • disruption of the protein matrix that surrounds starch granules is initiated by endopeptidases (proteinases) that are synthesized de novo in the aleurone during germination
      • The remaining storage material, starch, is present as highly crystalline granules that can be attacked by α-amylases, albeit very slowly at the temperatures used for germination. This enzyme is also synthesized de novo in the aleurone during germination and released into the starchy endosperm
  3. When fruit is ripening, it contains enzymes to break down cell walls
    • In the avocado, oil is mostly found in the idioblast cells which are distinguished by their large size and lignified walls (Platt-Aloia & Thomson, 1981; Werman & Neeman, 1987). These cell walls must be broken in order to release oil droplets before the oil can be separated. These cell walls can either be broken by mechanical force (mechanical grinding) (Werman & Neeman, 1987), enzymatic action on the cell walls of the idioblasts (Buenrostro & Lopez-Munguia, 1986), heat degradation, or any combination thereof. Avocado fruit naturally contains cell wall hydrolyzing enzymes that are present in high levels when the fruit is ripe. – “Addition of Exogenous Enzymes during Malaxing” Marie Wong, … Leandro Ravetti, in Green Vegetable Oil Processing, 2014
  4. Processing, such as by chewing, crushing, cooking, will break the cells walls to some extent and make the plant cell content available. This is probably why the cultural meme of “make sure to chew your food” happened at the same time as the push for plant based diets.

Finally, if the oil is extract from a plant, there is still a matter of whether that oil contains the lectins the plant made to deter predation; and there is the question of whether the oil contains mycotoxins and aflotoxins of fungi that may have been growing on the plant.

Side Note

Apart from microbiota, human mechanisms for breaking down food include enzymes and bile. Stomach acid (HCl) only serves to sterilize the food and activate pepsin (a protein breaking enzyme). For more, see https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookDIGEST.html

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